The present invention relates to a girth-welding process for a pipe and a high cellulose type coated electrode, which are suitable for coated arc welding of line pipes transporting CO.sub.2 -containing oil, CO.sub.2 -containing natural gas and CO.sub.2 gas. More particularly, the present invention relates to a girth-welding technique in which the weld metal exhibits excellence in preferential corrosion resistance, cold toughness and crack resistance.
In prior art, there have been published following literatures for proposing improvement with respect to preferential corrosion in weld zone of low alloy steel.
(1) The difference of Ni-content between the base metal and the weld metal has an influence on the local corrosion in the welded joint in the icy sea. (Takashi Abe and others; "TETSU-TO-HAGANE (Journal of the Iron and Steel Institute of Japan)" Vol. 72, (1986), Page 1266).
(2) The difference of Ni- and Cu-contents between the base metal and the weld metal have an influence on the local corrosion of the weld zone of the steel used in the icy sea and the degree of preferential corrosion depends upon a value, i.e., 3.8Cu+1.1Ni+0.3. (Kitaro Ito and others; "TETSU-TO-HAGANE" Vol. 72, (1986), p. 1265).
(3) A use of a low alloy welding rod containing Cu and Ni is effective in protecting a girth weld zone of a carbon steel pipe against preferential corrosion. (Hideaki Miyuki; "Zairyo (JOURNAL OF THE SOCIETY OF MATERIALS SCIENCE, JAPAN)" Vol. 38 (1989), No. 424, pp. 62-68).
(4) The Ni- and Mo-additives are effective in protecting a longitudinal seam weld zone of a welded steel pipe against preferential corrosion (Suga and others; Japanese Provisional Publication No.hei-3-170641/1991).
That is, for improving the preferential corrosion behavior of weld metal in an corrosion environment containing oxygen such as in the icy sea etc., the adding of Ni and Cu to the weld metal is proposed as an effective procedure, and the addition of Ni and Mo to the metal is presented for improving the preferential corrosion behavior in longitudinal seam weld zone of welded steel pipe.
Investigation, however has not yet been sufficiently made into inhibition of preferential corrosion in girth weld zone of line pipes for use in the corrosion environment containing CO.sub.2. That is, when a welded steel pipe or a seamless steel pipe is used for transportation of CO.sub.2 -containing oil, CO.sub.2 -containing natural gas or CO.sub.2 gas, there may sometimes occur preferential corrosion in weld zone such that a girth weld metal is preferentially corroded. This is because that there is difference in chemical composition and metallographic microstructure between the weld metal and the base metal, so that the weld metal portion is electro-chemically less noble and thus preferentially corroded.
In the prior art girth welding of the line pipes used in such the corrosion environment, neither method of girth welding nor high cellulose type welding rod has not been proposed with considering preferential corrosion, weldability and mechanical properties. In other word, if improved protection against preferential corrosion is aimed by adding Cu and/or Ni when a high cellulose type coated electrode is used, it is necessary to take account of weld cracks and mechanical properties of the weld metal. In spite of this situation, no high cellulose type coated electrode has been presented with considering the weld cracks and mechanical properties of the weld metal. In this connection, Japanese Provisional Publication No.sho-63-220994/1988 states that addition of Mo improves the mechanical properties, but there is no disclosure of the practical addition range, or the influence of the Mo addition upon the hot crack resistance and cold crack resistance.
It has been found that addition of Mo and Ni is effective to prevent a longitudinal seam weld zone from being preferentially corroded, but it has not been considered whether or not the addition of Mo functions as an inhibitor of the preferential corrosion around a girth weld zone. And no high cellulose type coated electrode with Mo added has been developed in view of practical use, with taking into account of the hardness and crack resistance of the weld metal.
In the practical environment in which line pipes are used, however, there often occur problems of the preferential corrosion caused by CO.sub.2. Therefore, it is strongly desired to develop a useful girth-welding process for a pipe and a high cellulose type coated electrode in order to improve the resistance to preferential corrosion, weldability and mechanical properties etc. in girth weld zone.